I have looked at the logs but didn't see anything. Maybe I am looking at th wrong logs, but I can't find it....
I looked at the error log in cpanel and all it shows is: client denied by server configuration: /home/XXuserXX/public_html/products/customer/register.php

Yes mod_security is installed.

the file permissions are the same as all other files 644

no I haven't changed the php.ini everything is the same.

The strange thing is I made a copy of the file with a different name, and if you use the different name everything works fine. It seems to be something with that filename that is causing the problem.